The 16-year-old Christchurch boy who drowned at the weekend was "well-mannered, popular and sporty", his school principal says.

Tominiko Junior Pelenato drowned on Saturday afternoon while swimming in the lake at the Pegasus Town development in North Canterbury. He was lost underwater for about 20 minutes as his friends and people nearby desperately tried to bring him to the surface.

Connor Roberts, a spokesman for the Todd Property Group which owns Pegasus, said the boy had been swimming with members of his church group when he got into difficulty and drowned.

Family and friends were said to be "in a state of shock". They gathered at the Pelenato household in Linwood yesterday to mourn his loss. Ngai Tahu placed a rahui (ritual prohibition) on the lake after the incident.

Catholic Cathedral College principal Bruce Henley described Tominiko as a "wonderful young man".

"We're just devastated that this has happened . . . he was a wonderful young man and he was popular with students and teachers".

Henley said Tominiko wore his school uniform with pride and "his tie was always straight".

"I took some flowers to the family house [yesterday] morning on behalf of the school board and students . . . and they are in a state of shock. . . . it really is devastating." Henley said staff and students would attend a special meeting this morning at the school where counselling and support would be offered.

Tominiko had just finished his third year at the school.

St John was alerted to the incident at 12.05pm and sent an ambulance and the Westpac rescue helicopter.

General manager of Garden City Helicopters, Simon Duncan, said paramedics were flying back from water-rescue training at Taylors Mistake when they were redirected to Pegasus after receiving the call from St John.

"The staff from the hire centre on the lake at Pegasus had taken out their inflatable boat to look for the boy . . . he had been under water for 20 minutes before he was recovered," Duncan said.

"It's a very unfortunate thing that has happened, and especially at this time of year, it really is awful."

A St John spokesman said Tominiko was pronounced dead at 12.39pm. The matter has been referred to the coroner, who will determine the cause of death.

Roberts said Todd Property Group had expressed its condolences to the Pelenato family.

"It's a terrible tragedy and being this close to Christmas makes it even more painful for family and friends."

Roberts said Tominiko and others had tried to swim from the island to the lake shore.

"Tominiko got into difficulty on the trip across and the others tried to help him but they weren't able to keep up above water."

The lake is 13 hectares and up to 7 metres deep in some parts, he said. "This is a reminder that any body of water carries risk . . . the lake is usually such a safe place but there is always a danger."

The supervisor of the Pegasus Flat White Cafe, who did not wish to have his name published, said the drowning had "affected everyone".

"It's extremely sad . . . we've been pretty quiet [yesterday] and a lot of people coming to swim in the lake . . . have been told they can't so they've gone somewhere else."